Tropomyosin-related kinase (TRK) is a receptor tyrosine kinase family of neurotrophin receptors that are found in multiple tissues types. Three members of the TRK proto-oncogene family have been described: TrkA, TrkB, and TrkC, encoded by the NTRK1, NTRK2, and NTRK3 genes, respectively. The TRK receptor family is involved in neuronal development, including the growth and function of neuronal synapses, memory development, and maintenance, and the protection of neurons after ischemia or other types of injury (Nakagawara, Cancer Lett. 169:107-114, 2001).
TRK was originally identified from a colorectal cancer cell line as an oncogene fusion containing 5′ sequences from tropomyosin-3 (TPM3) gene and the kinase domain encoded by the 3′ region of the neurotrophic tyrosine kinase, receptor, type 1 gene (NTRK1) (Pulciani et al., Nature 300:539-542, 1982; Martin-Zanca et al., Nature 319:743-748, 1986). TRK gene fusions follow the well-established paradigm of other oncogenic fusions, such as those involving ALK and ROS1, which have been shown to drive the growth of tumors and can be successfully inhibited in the clinic by targeted drugs (Shaw et al., New Engl. J. Med. 371:1963-1971, 2014; Shaw et al., New Engl. J. Med. 370:1189-1197, 2014). Oncogenic TRK fusions induce cancer cell proliferation and engage critical cancer-related downstream signaling pathways such as mitogen activated protein kinase (MAPK) and AKT (Vaishnavi et al., Cancer Discov. 5:25-34, 2015). Numerous oncogenic rearrangements involving NTRK1 and its related TRK family members NTRK2 and NTRK3 have been described (Vaishnavi et al., Cancer Disc. 5:25-34, 2015; Vaishnavi et al., Nature Med. 19:1469-1472, 2013). Although there are numerous different 5′ gene fusion partners identified, all share an in-frame, intact TRK kinase domain. A variety of different Trk inhibitors have been developed to treat cancer (see, e.g., U.S. Patent Application Publication No. 62/080,374, International Application Publication Nos. WO 11/006074, WO 11/146336, WO 10/033941, and WO 10/048314, and U.S. Pat. Nos. 8,933,084, 8,791,123, 8,637,516, 8,513,263, 8,450,322, 7,615,383, 7,384,632, 6,153,189, 6,027,927, 6,025,166, 5,910,574, 5,877,016, and 5,844,092).